use node-config module

I took the liberty of implementing this in the [rendr-cli](https://github.com/technicolorenvy/rendr-cli) and think it is a nice addition to the rendr-app-template project.

By using the node-config module, we can easily define a default config from which all other environment-specific configs can inherit from. While this PR is using yaml for the conf files, it is easy enough to swap out w/ .js or .json files, i chose yaml as it feels like a good sensible default.

Additionally, we are able to remove this file form the boiler-plate setup [server/env](https://github.com/airbnb/rendr-app-template/blob/master/server/lib/env.js)